Heretofore, limit switches are provided which are not to be passed by movable energy consumers or electric vehicles such as cranes, trolley carriages or the like, so that upon approach a predetermined distance relative to a given point is maintained by the crane. Such limit switches cost about 50 dollars and must be connected by additional lines to the electronic control system thus constituting an unnecessarily high expense for preventing movement of the crane past a given point.
It is therefore an object of the present invention to provide a more economical device for preventing overrunning of a predetermined point. This object is achieved in the manner that at least one of the energy or control contact lines is covered by an insulator which has a length that exceeds the braking path distance of the moveable energy consumer or electric vehicle. Such an insulator is generally about 0.2 to about 0.4 m long, depending on the stop or brake path of the crane, carriage trolley or the like, and costs about 25 to 50 cents, and therefore, only 1% or less of the price of a known limit switch.
The insulator is preferably arranged only on the face of the signal contact line and effectively causes the collector to disconnect from the contact line so that the moveable energy consumer will come to a stop.
In a further embodiment of the present invention, the insulator is made of a plastic having an electric surface resistance of at least about 10.sup.10 ohm and a shape of a U or T section which is undulated or ribbed in to increase the protection against leakage current.
The material of the insulator is selected preferably in accordance with the properties of the current collecting carbon so that suitable gliding properties are obtained and that wear of the insulator is rendered negligible. The insulator can be provided with resilient projections which snap into cuts or tapers provided in the grooves of a contact-line support or, if the contact line has a U-shaped or C-shaped cross-section along the outside of which the current collectors slide, the insulator can have a C-shaped cross-section which surrounds and is clamped to the contact line. To ensure the proper lifting or disconnection of the current collector from the contact lines, the insulator is preferably provided with beveled ends. Suitably the bevel is about 30.degree..
If there is the danger of the insulator being displaced on or from the contact line, it can be secured in position by gluing, for example, by the application of an adhesive tape which is first attached with the side having the greater adhesive force to the insulator and then secured on the contact line. If the insulator is removed or moved to a different location, the removal of the insulator will simultaneously result in the removal of the adhesive from the contact line.